The wind has for a long time been one of the sources of power of which man can avail himself. While not always present at a given site and while a widely varying force from interval to interval, the wind has been used to drive mills and pumps, and for many other purposes.
The better known form of windmill, the propeller-type, has a plurality of blades or vanes rotating about a horizontal axis. Such an arrangement can be quite efficient as long as the axis of rotation is aligned with the wind direction, but for practical utility the windmill must be provided with means for changing the direction of the axis as wind direction changes or must have "sails" of such size that a useful component of power can be derived even from winds of less favorable direction. Large propeller-type windmills cannot be built in areas where there is considerable turbulence, with quick changes in direction and force of the wind. Also, in populated areas the large propeller type may not be considered environmentally desirable, principally because of fear that the high-speed rotor blades may pose safety problems, and also because it has been discovered that the high-speed blades of giant windmills often create eddies in the wind that travel considerable distances, rattling windows in houses.
A second form of windmill, the turbine type, has buckets or blades arranged for unitary rotation about a central vertical axis, and hence is operated by wind from any direction. An early teaching of this, having two diametrically oriented buckets, is shown in Savonius U.S. Pat. No. 1,697,673. The structure is not so efficient as those with a horizontal axis, and also presents problems due to the use of only two blades. These problems include the presence of considerable vibration and the presence of considerable variation in starting torque for different angles of approach of the wind to the stationary rotor.
Attempts have been made to overcome the vibration problem by the use of a turbine having three blades symmetrically disposed about a central vertical axis. However, while more stable, the three blade turbines tended to be even more inefficient than the two blade turbine of the Savonius type. The Czernow French Pat. No. 727,519 shows a three bladed vertical axis wind turbine. Each blade is shown as having a curved portion and a planar portion; however, the straight portion of the blade is relatively short compared with the overall turbine dimensions. The Czernow turbine is rather inefficient principally because of the relatively short length of the planar portion of the blades.
Another teaching of a three bladed vertical axis wind turbine is the Norton U.S. Pat. No. 4,005,947. Norton illustrates embodiments of tubines having both two and three curved outer blades and correspondingly two or three curved inner blades. The outer blades have no straight or planar portion. The ends of the outer blades near the center of the rotor in the three blade configuration point almost straight at the rotor center, and are almost at right angles to the inner ends of the blades of my invention. The Norton invention is also not very efficient.
From the above teachings, it appeared as though the efficiency of the two blade turbine would have to be sacrificed if the stability and endurance of a three blade turbine was desired.